What DNA Reveals: Basics for Forensic and Family Genetics

DNA carries genetic information in most human cells. Forensic profiling now relies mainly on autosomal STRs (commonly 16-24 loci; CODIS uses 20 core loci in the U.S.), which provide highly discriminating profiles. Mitochondrial DNA and Y-STR testing support analysis of degraded samples and paternal lineages, respectively. Proper collection and lab methods are critical to reliable results.

What is DNA?

DNA (deoxyribonucleic acid) is the molecule that carries genetic information in almost all human cells. It sits mainly in the cell nucleus and encodes the instructions for physical traits and cellular function. Mature red blood cells are an exception: they lack a nucleus and therefore do not contain nuclear DNA.

Nuclear DNA vs. Mitochondrial DNA

Nuclear (autosomal) DNA comes in chromosomes and is inherited roughly half from each parent. Except for identical twins and very rare somatic mutations, a person's nuclear DNA profile is unique.

Mitochondrial DNA (mtDNA) lives in the cell's mitochondria and is inherited only from the mother. Because many copies of mtDNA exist per cell, mtDNA testing can succeed when nuclear DNA is degraded. However, mtDNA has less individual resolution than nuclear DNA and cannot distinguish between maternally related individuals.

There is also Y-chromosome DNA, passed from father to son, which is useful for tracing paternal lineages but does not distinguish male relatives on the same paternal line.

How forensic DNA profiling works today

Modern forensic labs most often analyze short tandem repeats (STRs). STRs are short, repeated DNA sequences that vary in repeat count between individuals. Labs measure the repeat lengths at multiple STR loci to build a DNA profile.

Profiles from autosomal STRs typically show one or two alleles per locus (one inherited from each parent). Laboratories compare profiles from a crime-scene sample to a reference sample and quantify how likely a random, unrelated person would match the profile.

In the United States, the FBI expanded the CODIS core loci to 20 STR markers in 2017; many jurisdictions use panels of roughly 16-24 autosomal STRs. Analysts separate and size STR alleles using methods such as capillary electrophoresis or, increasingly, massively parallel sequencing (also called next-generation sequencing) to get sequence-level detail.

When mtDNA or Y-STRs are used

Investigators use mtDNA when samples are old or highly degraded (hair shafts, skeletal remains) where nuclear DNA is insufficient. For male-specific lineage information or when mixed male/female DNA complicates interpretation, Y-STR testing can help, because it targets paternal-line markers.

Limitations and quality

Mixed samples, contamination, and very small amounts of DNA complicate interpretation. Laboratories follow strict protocols and statistical methods to report confidence in matches. Advances in sequencing and analysis continue to refine sensitivity and interpretation, but careful collection and laboratory practices remain essential.